Demands for controlling loads in electrical grids such as local grids are getting higher due to increased use of renewable energy suppliers with varying supply of energy. The varying energy supplied to the end users may prompt a selective switching-on and off of loads to keep the balance between supply and load. The loads can be controlled through so-called smart meters which receive information from the network provider or through a bus system used for building automations.
Conventionally, mechanical switches having relays or electronic switches having bipolar devices such as thyristors or TRIACs have been used. Mechanical switches generate noise when operating and are therefore not suitable as in-wall power sockets. They are typically located in central distribution boxes or switch cases. This reduces the flexibility for using bus-systems to control home appliances and increases the complexity of the wiring. Furthermore, mechanical switches have a limited total cycle of operation, are prone to vibrations, may influence other components when operated, and have a limited switching frequency.
Electronic switches using bipolar devices inherently exhibit power losses due the pn-junction having a voltage drop of about 0.7 V. The power losses associated with this inherent voltage drop require means for thermal dissipation which make such electronic switches unsuitable for in-wall sockets or for switches integrated into small spaces with restricted thermal dissipation.
Electronic switches employing high-voltage MOS-FETs might be an alternative since FETs have an ohmic current-voltage characteristic and have a low on-state resistance. However, conventional MOS-FETs or FETs using compensation structures requires large chip areas for carrying rated currents of 16 A which is a typical value for domestic installations. For example, each FET would need an active chip area of about 250 mm2 for a rated current of 16 A, an on-state resistance of 8 mΩ, and a rated blocking voltage of 650 V. Such devices would be too large for integrating on mounting rails or into an in-wall socket. The high rated blocking voltage of 650 V is needed since such high transients may occur in a phase of a 230 V network.
In addition to that, semiconductor switches are prone to over-currents and over-voltages, which might be caused by lightning strikes. Although measures are typically provided for discharging the over-currents and voltages, such measures may not be completely implemented leaving sections of TN-networks unprotected. Even when protected by residual-current circuit breaker (RCCB), which disconnects appliances when a fault current or a residual current occurs, the switches must be able to handle over-voltages and must carry the cut-off current according to the cut-off characteristic of the RCCB. In the event that the switch employing FETs is partially destructed due to a high cut-off current of an RCCB, the switch may be transferred into undefined conditions with large on-state resistances leading to increased thermal losses and therefore high risk of fire.
In view of the above, there is a need for improvement.